An optical temperature sensor is known that utilizes a temperature sensing device made of a semiconductor that changes its energy gap in response to changes in temperature (for example, see Patent Documents 1 through 5). This optical temperature sensor causes a signal light beam emitted from a first light emitting device and a reference light beam emitted from a second light emitting device to go through the temperature sensing device, and detects an external temperature based on the light intensity of each of the signal light beam and the reference light beam passing through the temperature sensing device.